Proper Duct Installation =

Professionally-Installed Comfort Delivery Ducts

Conditioned air is often lost in transition from comfort equipment to living spaces because of poorly designed duct layouts and poor installation practices, which cause homeowners to pay for conditioned air that never reaches the living spaces of their home. Poor duct layouts with overly long duct runs, numerous branches, and sharp bends, and poor installation with sagging, twisted, or crushed ducts, can severely limit air flow, reducing system performance and increasing energy costs. Professionally installed comfort delivery systems should be installed with compact duct layouts featuring short, straight duct runs with properly supported ducts, in accordance with industry standards. The ducts should be properly insulated, completely air sealed, and free from kinks and sharp bends that restrict air flow.

Alternate Terms

Optimum-Flow Comfort Delivery Duct Installation

Energy Saving Comfort Delivery Duct Installation

Professionally-Installed Comfort Delivery Ducts Sales Message

Professionally-installed comfort delivery ducts help ensure that heating and cooling can flow optimally to each room. This means less wasted energy along with enhanced room-by-room comfort. Wouldn’t you agree it’s important that your comfort delivery system is installed to deliver optimum performance?

Climate

For best performance, install ducts within the home’s thermal envelope. If installed in an uninsulated attic in dry climates, ducts may be laid on the ceiling deck. In the humid, mixed-humid, and marine climates, the ducts must be encapsulated in closed-cell spray foam if installed on the ceiling deck. Or, ducts can be hung from the attic ceiling with strapping or supported with saddle supports. In humid climates, ducts in vented attics should be hung or supported in the hottest part of the attic and contact of the ducts with other objects in the attic should be avoided to reduce the possibility of condensation forming on the ducts.

Description

Flexible duct, known as flex duct, is very convenient ducting for attaching supply air outlets to rigid trunk ductwork, and, in many cases, flex duct comprises the entire duct system. Flex duct made for HVAC applications is typically constructed of a plastic inner liner attached to a metal wire helix (or coil) to make round flex duct. The duct comes with a layer of fiberglass blanket insulation already attached around the duct. The insulation is covered and protected by a polyethylene or foil vapor barrier. Flex duct is typically available in insulation values of R-4, R-6, and R-8. For residential HVAC systems, insulated flex duct typically comes in diameters of 4 inches through 10 inches; above 10 inches, it comes in even sizes of 12, 14, 16, etc., up to 22 inches measured at the radius of the metal helix.

Figure 1 - Flex duct consists of a plastic inner liner attached to a metal coil, covered by a layer of fiberglass blanket insulation, which is covered by foil or plastic vapor barrier. (Image Courtesy of Calcs Plus)

Ideally, ducts should be located inside conditioned space, i.e., within the thermal envelope of the home. This could be in a dropped ceiling duct chase; in an insulated and air sealed attic, basement, or crawlspace; or between the floors of a two-story home. However, ducts are often installed in uninsulated, unsealed crawlspaces or attics that are outside the thermal envelope or conditioned space of the home. In these locations, the ducts must be thoroughly air sealed and insulated and properly installed and supported to avoid air flow losses, energy losses, and moisture problems.

In attics in dry climates, ducts can be laid on the ceiling joists or ceiling deck and covered with blown insulation (Figure 2). In humid or mixed-humid climates, ducts cannot be laid on any flat surface (such as the ceiling deck or ceiling joists whose cavities are filled with insulation) because of concerns about the formation of condensation on the outside of the ducts under the right dew point conditions. The only exception to this is if the ducts are first sealed, insulated, and completely encapsulated in at least 1.5 inches of closed-cell spray foam (Figure 3 Shapiro et al. 2013). Building America recommends, and DOE Zero Energy Ready Home requires, that in all climates, if ducts are to be “buried,” they must first be insulated with R-8 duct insulation then encapsulated with at least 1.5 inches of closed-cell spray foam insulation, then covered with at least 2 inches of blown insulation.

If the buried and encapsulated duct technique cannot be accommodated, then the ducts are typically hung from the attic ceiling with strapping or supported in saddles anchored to the attic framing. The flex duct must be adequately supported to ensure the duct is not crimped and to maintain both air flow through the duct and the integrity of the duct coil and insulation material. When the flex duct is improperly supported, for example by spacing the support straps or saddles too far, the ducts can sag, restricting air flow. When the support straps are too narrow or too tight, the metal helix or plastic inner liner of the flex duct can be crushed or damaged, restricting air flow and compromising the insulation. If the insulation is overly compressed, cold spots can occur on the exterior of the duct where condensation can form in hot, humid attics. Crushed ducts can eventually fail causing air leakage and large energy losses. Hanger straps or saddles need to be wide enough to prevent the straps from compressing the insulation and cutting into the inner liner. Straps or saddles must be adequately spaced to prevent sagging of the ducts.

In hot-humid climates, if ducts are to be installed in an uninsulated, vented attic, hang the ducts in the hottest part of the attic to maintain the surface temperature of the ducts above the dew point and minimize contact of the ducts with other objects in the attic (Bailey and Bailey 2013).

How to Properly Support Flex Duct that is Hung in an Attic or Crawlspace

1. Support flex duct using straps or saddle supports or as specified by the manufacturer. Hanger straps in contact with flex duct should be at least 1.5 inches wide, preferably wider, (ADC 2010) and should be sufficiently loose to prevent restriction of the internal diameter of the duct when the weight of the supported section rests on the hanger or saddle material. Factory-installed suspension systems integral to the flexible duct are an acceptable alternative hanging method when the manufacturer’s recommended procedures are followed. Ducts can also be supported by a saddle-shaped support held up by a pole that is attached to attic framing. The supporting width of the saddle must exceed 5.5 inches along the length of the duct (ICC-ES 2007).

2. Space supports at 4-foot intervals or closer to ensure that the flexible duct does not sag more than 1/2 inch per lineal foot between the supports.

Supports should be installed at least every once 4 feet. The maximum allowable sag in flexible duct is one-half inch per foot. (Image Courtesy of Calcs Plus)

3. Place supports at each connection between flex duct and rigid metal duct. For example, in Figure 6 a hanger is placed at the trunk line where a flex connection is made.4. Ensure that strapping, framing, or other supports are in place to support all air devices (such as diffuser boxes, register boxes, or grille boxes) and all duct peripherals (such as junction boxes, wyes, plenums, etc.).5. Support the flex duct between a metal connection and a bend by allowing the duct to extend straight for a distance equal to or greater than the radius of the flex duct being installed before making a bend. The support will hold the duct straight for an adequate distance from the connection to avoid possible damage to the flex duct by the metal edge of the collar.

Ensure that hanger straps or other supports are in place to support the flex duct between a metal connection and a bend, to support trunk lines at a branch duct connection, and to support supply registers and other duct connections. (Image Courtesy of Calcs Plus)

6. Support duct before and after sharp bends (see figure below).

Install duct supports before and after any sharp bends in ducts. (Image Courtesy of Calcs Plus)

7. Support any vertically installed sections of flex duct with support straps installed at no more than 6 feet apart.

Ensuring Success

Plan for duct layout in the initial plans so that framing obstacles can be minimized. Install ducts within the thermal boundary of the home if possible. If installed in an uninsulated attic in dry climates, ducts can be laid on the ceiling deck and buried in blown insulation. In humid climates, encase the ducts in closed-cell spray foam. If ducts are hung, inspect that ducts are supported at least every 4 feet with strapping that is at least 1.5 inches wide (wider is better) and with no more than ½ inch of sag per lineal foot between supports.

Scope

Duct Distribution Quality Installation

If ducts are to be installed in a vented, uninsulated attic or crawl space, hang the ducts with supporting straps or saddles of sufficient width and frequency to adequately support the ducts so that the ducts do not sag and are not pinched or compressed in a manner that would cause damage to the ducts, the duct insulation, or the duct air barrier covering.

The ducting should be supported at the manufacturer’s recommended intervals, but the supports should be no greater than 4 feet apart and there should be no more than ½ inch sag per foot between the supports. A connection to rigid duct or equipment is considered a support joint.

Long horizontal duct runs with sharp bends should have additional supports before and after the bend.

Supports should be at least 1.5 inches wide.

Straps should not compress the inner core or constrict air flow. The supports should not excessively compress the vapor barrier and insulation material because compressing the insulation could lead to condensation at that point.

Factory-installed support systems that are part of the flex duct can be used if manufacturer’s procedures are followed.

Flexible ducts may rest on ceiling joists or truss supports, if the manufacturer’s support spacing guidance is followed.

A support should be installed between a metal connection and a bend and the duct should be allowed to extend straight for at least one duct diameter before making the bend.

Vertically installed ducts shall be supported every 6 feet at a minimum.